This invention relates to low noise amplifiers and more particularly to a distributed low noise amplifier. The amplifiers may typically, but not exclusively, find application in radio telescope applications.
The current trend in radio telescopes is to build telescopes with high sensitivity over broad bandwidths. To reach high sensitivities, very large collection areas and ultra low noise amplifiers are required. The large collection area comprises hundreds of smaller receivers, each having a respective low noise amplifier (LNA). The LNA must therefore not only have a low noise figure over a broad bandwidth, but must also be linear over a large dynamic range and not be very expensive.
To the best of the applicant's knowledge, almost all the LNA's used in radio telescopes are harmonic LNA's. These harmonic LNA's comprises a single transistor in the first amplification stage. At each frequency, the transistor has a minimum noise figure, Fmin, which is achieved when the transistor is connected to an input impedance Zopt. An impedance matching circuit is therefore required to match the amplifier's input impedance to the transistor's optimum low noise impedance, Zopt. Harmonic amplifiers have two main disadvantages. Firstly, due to the harmonic nature of the impedance matching circuit, the LNA's low noise performance degrades over large bandwidths and secondly, the impedance matching circuit also results in signal loss, insertion loss and additional noise generated by the circuit.
Another well-known amplifier arrangement is the distributed amplifier. Distributed amplifiers are capable of very large amplification bandwidths and have better linearity and dynamic range than harmonic amplifiers. They are therefore ideally suited as broadband amplifiers in radio telescopes, except for their noise figure. It is well known that distributed LNA's (DLNA) have a higher noise figure than harmonic LNA's.
A distributed amplifier comprises an input transmission medium with an input for the amplifier at one end thereof, an output transmission medium with an output for the amplifier at one end thereof and a number of amplifier parts, with the input of each amplifier part connected to the input transmission medium and the output of the amplifier part to the output transmission medium. The input transmission medium, together with the input impedance of the amplifier parts, which is normally capacitive, form a transmission line. When a signal is applied to the amplifier, it propagates along the input transmission medium. As the signal passes each part, it is amplified and added to the output transmission medium.
In a normal distributed amplifier, the amplified signals are added in phase on the output transmission medium. In other words, the signals from each amplifier part arrive at the same time at the output of the amplifier. The time delay from the input, through each amplifier part, to the output is the same for each part (that is when the difference is much less than the period of the input signal). For an ideal distributed amplifier, having n parts, each with a voltage gain of Av1, the total power gain is A∝(Av1n)2. With the ideal, it is assumed that no other component, besides the amplifier parts, generate noise and that there is no signal loss on the transmission mediums.
However, each amplifier part also generates noise that is transferred to the output transmission medium. If each part transfer has a noise power of N1,o, the total noise added tot the output transmission medium is No=N1,on, because the noise of the amplifier parts is uncorrelated. The noise figure (or noise-signal ratio) therefore decreases inversely to the number of parts No/A∝1/n.
But some noise generated by each part is also transferred to the input transmission medium. This input noise is then amplified by the other parts and added to the output transmission medium, similar to the signal, giving an amplified input noise N1,i∝n2 at the output. The total amplified input noise is then Ni∝n3 for many parts.
Therefore, as the number of part increases, the output noise-signal ratio decreases, but the amplified input noise-signal ratio increases. A distributed amplifier has therefore an optimum number of parts, for which the total noise-signal ratio is a minimum.
There is some correlation between the input and output noise of an amplifier part, because the noise is generated by a common noise source. Let C=Cre+jCim be the complex correlation coefficient. When the delay through the amplifier parts is the same, the phase of the correlation between the correlated part of the output noise N1,o and amplified input noise N1,i is the same as the phase between the input and output noise. Only the real part (in-phase part) of C then gives noise cancellation, so that the minimum noise figure of an ideal distributed amplifier is proportional to 1−Cr.
By using the right input impedance in harmonic LNA's, Zopt, the input noise is reflected back to the transistor, such that the correlated part is in phase with the output noise. The amplified input noise then cancels a large part of the output noise, resulting in a noise figure proportional to √{square root over (N1,jN1,o)}(√{square root over (1−Cim2)}−Cre). For High Electron Mobility Transistors (HEMT) used in noise amplifiers, Cim is close to 1, so that the noise of a harmonic amplifier is much lower than the noise of a distributed amplifier.